was litterally the first app i downloaded when i got an android device 3 years ago and i've not stopped using it since. it has live data tracking, and also tracks time, and can alert you when your approaching data limits, and other such things. quite useful, can be a little bit confusing at times but play around with it for a bit and it'll work wonders. also the widget in notification bar to tell you your data usage quickly is nice and useful.